In lean burn engines, it is necessary under certain engine conditions to change the fuel calibration from stoichiometric AFR (air to fuel ratio) to lean AFR or vice-versa. This may occur during driving when the engine speed/load operating point is moved into or out of a lean calibration window, and during lean cruise conditions when the engine AFR has to be perturbed briefly back to a rich AFR at regular intervals in order to purge a NOx trap in the exhaust system. The latter purge sequence could be very frequent, typically a 1 second rich excursion is required for every 30 seconds of lean cruise running.
A well known control problem with lean burn engines is that the AFR change can cause torque fluctuations which are unacceptable for driveability. This arises from the fact that the intake air mass drawn into the engine is fixed and is set by the driver's pedal position at a given vehicle speed. If the AFR calibration is to be suddenly changed against this fixed air mass, the fuel mass will change affecting the energy produced and the engine torque. For example, a change in AFR from stoichiometric to 22:1 represents a 35% drop in output torque at the same air mass.
To compensate for this sudden change, the fundamental requirement is that the intake air mass must in some way be changed at the same time as the AFR is changed, so that the fuel mass in the engine remains substantially the same before, during and after the AFR change.
In one way of achieving this in the prior art, it is left to the driver to respond to the perceived change in torque by moving the demand pedal to a new position to change the intake air mass thereby regaining the engine torque. In effect, the driver response is in this case built into the control loop, but this is only acceptable for small excursions in the engine torque.
In another method disclosed in the prior art, an electrically controlled throttle (ETC) is used to isolate the driver from direct interface with the engine throttle. The driver sets the torque demand with a potentiometer which the ETC translates into a throttle position precisely matching the air mass required before and after the AFR change. Thus during the AFR change, while the ETC rapidly moves the throttle from one position to a new position to change the intake air mass, the driver who sets the torque demand does not feel any change in the engine torque and therefore need not adjust his demand pedal position. This AFR change, being totally transparent to the driver, is then termed a seamless transition.
In WO96/21097, it is proposed to use an air dilution throttle in parallel with the main throttle and to gang the two throttles together to move at all times at the same throttle angle. An on/off valve is provided in series with the air dilution throttle to enable or disable the air dilution flow according to the lean or stoichiometric mode, respectively. This switching method has been demonstrated to produce seamless transitions similar to those using ETC, and has advantages over ETC in that the ganged throttles are permanently connected to the demand pedal giving the driver direct control. Such a system has advantages of lower cost and higher reliability over the ETC system. It also lends itself particularly well to the operating sequence of purging an NOx trap by briefly flicking the on/off valve without moving the main throttle.
While the use of parallel throttles and an on/off valve in series with one of the two throttles is effective, it has disadvantages in that the air dilution throttle is a potential source of additional air leakage when both throttles are closed during engine idle operation, during which only a very small amount of air leakage is permissible. This has resulted in increased technical difficulties in the design of the throttles because, even in the case of a single throttle, the total air leakage can attain a critical level. Furthermore other design considerations, in addition to the control of air leakage, for example, throttle effort, sludge and ice protection, fail-safe regulations etc., that is applied to the main throttle must equally be applied to the air dilution throttle.